Alpha-radiolysis of nitric acid aqueous solution irradiated by ²³⁸Pu source

Alpha-radiolysis of nitric acid aqueous solution irradiated by 238 Pu source 1 first-author 1 2 2 1 3 corresp Corresponding author. E-mail address: gelin@ustc.edu.cn Corresponding author. E-mail address: gelin@ustc.edu.cn gelin@ustc.edu.cn School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China; School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230026, China; China Institute of Atomic Energy, Beijing 102413, China; China Institute of Atomic Energy, Beijing 102413, China; Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei 230031, China Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei 230031, China Alpha radiolysis of nitric acid aqueous solution by a 238 Pu source is investigated experimentally and theoretically. The time dependence of the nitrous acid yield on dose rate, nitric acid concentration, and nitrate ion concentration is studied. A novel kinetic model for the α-radiolysis of nitric acid aqueous solution is established, by considering the direct and indirect effects,. The simulation results agree well with the experimental data, indicating the validity of our model to treat the reaction paths for generation and consumption of nitrous acid. It is shown that the redox reactions involving Pu cannot be neglected in the α-radiolysis of the solution. The results provide a better understanding of the α-ray radiolysis of aqueous nitric acid. α-radiolysis Nitric acid Plutonium Aqueous solution Kinetic model Nitrous acid 1. Introduction 1 Nitric acid is the most common inorganic acid used in the PUREX (Plutonium Uranium Recovery EXtraction) process, under intense α, β and γ radiations [ 1 1 - 4 4 1-4 ]. The radiolytic products of aqueous nitric acid, such as nitrous acid, hydrogen peroxide, hydrogen and oxygen affect the organic phase degradation, valence of actinides, corrosion of the container, and the reductants in PUREX [ 4 4 , 5 5 ]. Thus investigation on the radiolysis of nitric acid attracts increasing attention [ 5 5 - 10 10 5-10 ]. Experiments on radiolysis of solid nitrates were began in 1970s [ 11 11 - 15 15 11-15 ], and the direct radiolysis effect was proposed. For nitric acid aqueous solution, the reactions involved are more complex due to the influence of water radiolysis. The formation of gases (e.g., H 2 and O 2 ) in the solution was investigated [ 15 15 - 18 18 15-18 ]. It was proved that both the direct radiolysis of solutes and the reactions between radiolytic products and water contributed to the production of gases. However, while there are many experimental and theoretical works on γ-ray radiolysis of aqueous nitric acid [ 7 7 - 9 9 7-9 , 19 19 - 21 21 19-21 ]., just limited investigations using an α-radiation source have been reported [ 16 16 - 18 18 16-18 , 22 22 - 25 25 22-25 ], largely due to the lack of an α irradiation facility and the operation difficulties. The yield of HNO 2 was measured by Miner et al . with the 238 Pu microsphere as an α-radiation source [ 26 26 ]. The survey on HNO 2 yield is important for understanding the mechanism of α radiolysis of aqueous nitric acid and for the application in PUREX, considering that HNO 2 can consume the reductant for Pu(III) in PUREX. However, the concentration gradient of nitric acid in Ref.[ 22 22 ] is too large to be applied for practical use. On the theoretical side, the redox reactions about Pu and some radiolytic products of nitric acid were proposed in Refs. [ 27 27 , 28 28 ], though the primary yields of some radicals and several reactions involving nitric acid were absent. To the best of our knowledge, there is still no complete kinetic model for the α-radiolysis of aqueous nitric acid. In this paper, experiments on is conducted production of NO 2 − using 238 Pu as the α-radiation source ( 238 Pu decays, in a half-life of 87.7 years, by emitting α particles in averaged energy of 5.49 MeV). A novel complete kinetic model for α-radiolysis of aqueous nitric acid is established using MCPA FACSIMILE Version 4.0 (MCPA Software Ltd., UK) [ 29 29 ]. FACSIMILE is a powerful modeling tool designed to efficiently solve differential equations in engineering and scientific researches, and is widely used in chemical kinetic simulation [ 30 30 , 31 31 ]. The experimental and simulation results should benefit for a better understanding of the radiolysis of aqueous nitric acid in nuclear industry, since the concentration of nitric acid in our work is within the range of that in the typical PUREX process. 2. Materials and Methods 1 2.1. Materials 2 The 238 Pu in the form of PuO 2 powder and the deionized water (18.25 MΩ · cm) were provided by China Institute of Atomic Energy.. The nitric acid and sodium nitrate were analytical pure (Sinopharm Group Co. Ltd.). 2.2. Preparation of stock solution 2 A 238 Pu solution， instead of the 238 Pu microsphere [ 26 26 ], was employed, in consideration of radiation uniformity, convenience of dose rate determination (nearly all radiation energy was absorbed by the solution), and the less amount (just mg or less) of α emitter required for the experiments [ 26 26 ]. The long-term stock solution was prepared by dissolving 238 PuO 2 powder in aqueous nitric acid catalyzed by Ag + -Ag 2+ cycle under constant-current electrolysis. The 238 Pu(IV)-NO 3 − stock solution was purified with No. 2606 anion exchange resin packed column (offered by China Institute of Atomic Energy). During the purification, Pu was adsorbed in the exchange column and the Ag + remained with the solution. The Pu was eluted by 0.35 M HNO 3 solution. It is worth noting that there should be radiolytic resin products in solution, since the Pu is a strong radiation source. Thus, before the experiments, the small amount of stock solution to be used was purified again with the same method. In the second purification, the radiation source was less and the purification time was shorter, with negligible radiolytic products. 2.3. Preparation of experimental solutions 2 The stock solution was adjusted to desired conditions. Different dose rates were achieved by adjusting the Pu concentration, noted as c (Pu), and the variation of NO 3 − concentration was realized by adding sodium nitrate. The concentration of nitric acid used in the experiments was within the range of PUREX process (about 1.0–5.0 M), thus the results are suggestive for understanding of the actual α-radiolysis of aqueous nitric acid. 2.4. Characterization 2 The concentration of nitrous acid was determined by spectrophotometry (SPECORD S600 Spectrophotometer, Analytik Jena AG, German), using a 1-cm quartz cuvette (YiXing JingKe Optical Instrument Co. Ltd.). Spectrophotometry was used to continuously measure the production of nitrous acid. 2.5 Kinetic model 2 There are many reaction paths for the α-radiolysis of aqueous nitric acid. Firstly, the radiolytic products of water react with nitric acid or nitrate ion [ 30 30 ]: <math display="block" id="M1"><mrow><msup><mrow/><mtext>•</mtext></msup><msub><mrow><mtext>OH+HNO</mtext></mrow><mtext>3</mtext></msub><msup><mtext>→</mtext><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O, </mtext><mi>k</mi><mtext>=1</mtext><mtext>.4</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>8</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> <math display="block" id="M2"><mrow><msup><mrow/><mtext>•</mtext></msup><msub><mrow><mtext>H+NO</mtext></mrow><mtext>3</mtext></msub><msup><mrow/><mtext>-</mtext></msup><msup><mtext>→</mtext><mtext>•</mtext></msup><msub><mrow><mtext>HNO</mtext></mrow><mtext>3</mtext></msub><msup><mrow/><mtext>-</mtext></msup><mtext>, </mtext><mi>k</mi><mtext>=1</mtext><mtext>.0</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>7</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> The effect of direct radiolysis of solute also plays an important role for concentrated solution, as the solute absorbs a large fraction of the radiation energy [ 21 21 ]: <math display="block" id="M3"><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub><msup><mrow/><mtext>-</mtext></msup><msub><mrow><mtext>-\/\/\→NO</mtext></mrow><mtext>2</mtext></msub><msup><mrow/><mtext>-</mtext></msup><mtext>, O</mtext><mrow><mo>(</mo><mrow><msup><mrow/><mtext>1</mtext></msup><mtext>D</mtext></mrow><mo>)</mo></mrow><mtext>, O</mtext><mrow><mo>(</mo><mrow><msup><mrow/><mtext>3</mtext></msup><mtext>P</mtext></mrow><mo>)</mo></mrow></mrow></math> <math display="block" id="M4"><mrow><mtext>O</mtext><mrow><mo>(</mo><mrow><msup><mrow/><mtext>1</mtext></msup><mtext>D</mtext></mrow><mo>)</mo></mrow><msub><mrow><mtext> + H</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>O→H</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>2</mtext></msub></mrow></math> <math display="block" id="M5"><mrow><mtext>O</mtext><mrow><mo>(</mo><mrow><msup><mrow/><mtext>3</mtext></msup><mtext>P</mtext></mrow><mo>)</mo></mrow><msub><mrow><mtext>+ NO</mtext></mrow><mtext>3</mtext></msub><msup><mrow/><mtext>-</mtext></msup><msub><mrow><mtext>→NO</mtext></mrow><mtext>2</mtext></msub><msup><mrow/><mtext>-</mtext></msup><msub><mrow><mtext>+O</mtext></mrow><mtext>2</mtext></msub></mrow></math> For concentrated nitric acid solution, radiolysis of the un-dissociated nitric acid molecule should not be neglected [ 20 20 , 21 21 ]: <math display="block" id="M6"><mrow><msub><mrow><mtext>HNO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>-\/\/\→HNO</mtext></mrow><mtext>2</mtext></msub><mtext>,O</mtext><mrow><mo>(</mo><mtext>D</mtext><mo>)</mo></mrow><mtext>,O</mtext><mrow><mo>(</mo><mtext>P</mtext><mo>)</mo></mrow></mrow></math> <math display="block" id="M7"><mrow><msub><mrow><mtext>HNO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>-\/\/\→NO</mtext></mrow><mtext>3</mtext></msub><msup><mrow><mtext>+ e</mtext></mrow><mtext>-</mtext></msup><msub><mrow/><mrow><mtext>aq</mtext></mrow></msub><msup><mrow><mtext>+H</mtext></mrow><mtext>+</mtext></msup></mrow></math> In addition, the actinides can react with radicals in aqueous nitric acid [ 27 27 , 35 35 ]. The redox reactions involving actinides may also affect the generation and consumption of nitrous acid, for example [ 27 27 ]: <math display="block" id="M8"><mrow><mtext>Pu</mtext><mrow><mo>(</mo><mrow><mtext>III</mtext></mrow><mo>)</mo></mrow><msup><mtext>+</mtext><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>2</mtext></msub><mtext>→Pu</mtext><mrow><mo>(</mo><mrow><mtext>IV</mtext></mrow><mo>)</mo></mrow><msub><mrow><mtext>+NO</mtext></mrow><mtext>2</mtext></msub><msup><mrow/><mtext>-</mtext></msup><mi/><mi>k</mi><mtext>=2</mtext><msup><mrow><mtext>.5×10</mtext></mrow><mrow><mtext>3 </mtext></mrow></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> Therefore, in order to get an overall picture about the radiolysis of aqueous nitric acid under α-radiation, we set up a kinetic model including the primary yields of the radicals for the radiolysis of both pure water and nitric acid, and the reaction sets for the reaction between these radicals and nitric acid, water, and Pu. The radical yields of pure water, nitric acid, and nitrate ion are essential to the simulation for α-radiolysis of nitric acid aqueous solution. The primary yields for the α-radiolysis of pure water at α-particle energy of 4.6 MeV were reported by Eriksen et al ., [ 40 40 ]. This energy is close to the averaged α-particle energy for 238 Pu (5.5 MeV), so the radical yields in Ref. [ 40 40 ] were used in this work with slight adjustment. Due to the rapid scavenging reaction between pre-solvated electron and nitrate ion, NO 3 2− should replace e − aq . LET (linear energy transfer) effect was also considered. The radical yields for pure water are given in Table 1 Table 1 . The γ-radiolysis of nitric acid/nitrate ion was studied by Jiang et al. [ 22 22 ]. For α-radiolysis, the decomposition paths should be the same and the radical yields should be smaller. The radical yields used in this work (the right rows of Table 1 Table 1 ) are adjusted by fitting with experimental data. 4 1 Radiolysis of pure water 2 1 Radiolysis of un-dissociated nitric acid 2 1 Radiolysis of nitrate ion 1 1 Radicals 1 1 G value 1 1 Radicals 1 1 G value 1 1 Radicals 1 1 G value 1 1 Radicals 1 1 G value 1 1 G H2O (H • ) 1 1 0.21 1 1 G H2O ( • HO 2 ) 1 1 0.22 1 1 G HNO3 (HNO 2 ) 1 1 0.5 1 1 G NO3- (NO 2 − ) 1 1 0.7 1 1 G H2O (H 2 ) 1 1 1.23 1 1 G H2O ( • NO 3 2- ) 1 1 0.2 1 1 G HNO3 ( • O(P)) 1 1 0.5 1 1 G NO3- ( • O(P)) 1 1 0.7 1 1 G H2O ( • OH) 1 1 0.24 1 1   1 1   1 1   1 1   1 1   1 1   The yield of certain radical in solution is described by: <math display="block" id="M9"><mrow><msub><mi>G</mi><mrow><mi>t</mi><mi>o</mi><mi>t</mi></mrow></msub><mrow><mo>(</mo><mrow><msub><mi>X</mi><mi>i</mi></msub></mrow><mo>)</mo></mrow><mo>=</mo><munder><mstyle mathsize="140%" displaystyle="true"><mo>∑</mo></mstyle><mi>j</mi></munder><msub><mi>G</mi><mrow><msub><mi>Y</mi><mi>j</mi></msub></mrow></msub><mrow><mo>(</mo><mrow><msub><mi>X</mi><mi>i</mi></msub></mrow><mo>)</mo></mrow><msub><mi>f</mi><mi>j</mi></msub><mo>,</mo></mrow></math> where, X i is the i th radical; Y j is the j th component (water, molecular nitric acid, nitrate ion, etc.) of solution; and f j = N j / N tot is the electron fraction of that component, with N j and N tot being the electron number of the corresponding component and total electron number of the solution, respectively. The absorbed energy for corresponding component ( E j = E tot f j ) varies as the concentration of solute changes. E tot represents the total energy absorbed by the solution. Thus the primary radical yield varies as the concentrations of solutes change. In order to build a kinetic model, the reaction sets and corresponding rate constants about water, nitric acid and Pu are needed. The reactions between radicals produced in pure water radiolysis have been studied by many researchers. Buxton et al . summarized the reactions and determined the selected values for reaction rate constants Ref. [ 32 32 ]. Elliot and Bartels reported the reaction rate constants in water radiolysis with temperature from room temperature to 300℃ based on the experimental data in Chalk River Laboratory [ 38 38 ]. The reactions about pure water radiolysis can be obtained from the two works. There are various nitrogenous radicals and corresponding reactions in the radiolysis of nitric acid aqueous solution [ 39 39 - 42 42 39-42 ]. In Pu+HNO 3 system, reactions involving Pu have been studied by Vladimirova [ 27 27 , 28 28 ]. All the reactions and rate constants used in this model are summarized in Table 2 Table 2 . 1 1 No. 1 1 Reaction 1 1 Rate constant 1 1 No. 1 1 Reaction 1 1 Rate constant 1 1 R001 [ 32 32 ] 1 1 e aq − + H 2 O→H • + OH − 1 1 1.9×10 1 1 1 R113 [ 51 51 ] 1 1 • OH + • NO 2 →ONOOH 1 1 4.5×10 9 1 1 R002 [ 32 32 ] 1 1 e aq − + e aq − →H 2 + 2OH − 1 1 5.5×10 9 1 1 R114 [ 39 39 ] 1 1 H 2 O 2 + HNO 2 →ONOOH + H 2 O 1 1 7.17×10 5 1 1 R003 [ 32 32 ] 1 1 e aq − + H • →H 2 + OH − 1 1 2.5×10 10 1 1 R115 [ 39 39 ] 1 1 ONOOH + H 2 O→H 2 O 2 + HNO 2 1 1 3×10 2 1 1 R004 [ 32 32 ] 1 1 e aq − + • OH→OH − 1 1 3.0×10 10 1 1 R116 [ 42 42 ] 1 1 ONOOH →H + + NO 3 − 1 1 0.9×10 0 1 1 R005 [ 32 32 ] 1 1 e aq − + • O − →2OH − 1 1 2.2×10 10 1 1 R117 [ 39 39 ] 1 1 • H + • NO 2 →H + + NO 2 − 1 1 1.0×10 10 1 1 R006 [ 32 32 ] 1 1 e aq − + H + →H • 1 1 2.3×10 10 1 1 R118 [ 39 39 ] 1 1 e aq − + • NO 2 →NO 2 − 1 1 1.0×10 10 1 1 R007 [ 32 32 ] 1 1 e aq − + H 2 O 2 →OH − + • OH 1 1 1.1×10 10 1 1 R119 [ 32 32 ] 1 1 e aq − + NO 2 − →NO 2 2− 1 1 3.5×10 9 1 1 R008 [ 32 32 ] 1 1 e aq − + HO 2 − →2OH − + • OH 1 1 3.5×10 9 1 1 R120 [ 51 51 ] 1 1 • H + NO 2 − → • NO + OH − 1 1 1.6×10 9 1 1 R009 [ 32 32 ] 1 1 e aq − + O 2 →O 2 •− 1 1 1.9×10 10 1 1 R121 [ 32 32 ] 1 1 • OH + • NO 2 2− →NO 2 − + OH − 1 1 3.0×10 9 1 1 R010 [ 32 32 ] 1 1 e aq − + O 2 •− →O 2 2− 1 1 1.3×10 10 1 1 R122 [ 32 32 ] 1 1 H + + • NO 2 2− → • NO + OH - 1 1 2.0×10 10 1 1 R011 [ 32 32 ] 1 1 H • + H 2 O→H 2 + • OH 1 1 1.0×10 1 1 1 R123 [ 32 32 ] 1 1 H 2 O + • NO 2 2− → • NO + 2OH − 1 1 1.0×10 5 1 1 R012 [ 32 32 ] 1 1 H • + H • →H 2 1 1 7.8×10 9 1 1 R124 [ 42 42 ] 1 1 ONOOH→H + + ONOO − 1 1 5.0×10 4 1 1 R013 [ 32 32 ] 1 1 H • + • OH→H 2 O 1 1 7.0×10 9 1 1 R125 [ 42 42 ] 1 1 H + + ONOO − →ONOOH 1 1 5.0×10 10 1 1 R014 [ 32 32 ] 1 1 H • + OH − →e aq - + H 2 O 1 1 2.2×10 7 1 1 R126 [ 52 52 ] 1 1 2HNO 2 → • NO + • NO 2 + H 2 O 1 1 1.34×10 1 1 1 R015 [ 32 32 ] 1 1 H • + H 2 O 2 → • OH + H 2 O 1 1 9.0×10 7 1 1 R127 [ 52 52 ] 1 1 • NO + • NO 2 + H 2 O →2HNO 2 1 1 1.68×10 8 1 1 R016 [ 32 32 ] 1 1 H • + O 2 →HO • 2 1 1 2.1×10 10 1 1 R128 [ 42 42 ] 1 1 ONOOH→ • NO 2 + • OH 1 1 3.5×10 −1 1 1 R017 [ 32 32 ] 1 1 H • + HO • 2 →H 2 O 2 1 1 1.14×10 10 1 1 R129 [ 39 39 ] 1 1 HNO 2 + HNO 3 →2 • NO 2 + H 2 O 1 1 4.5×10 −2 1 1 R018 [ 32 32 ] 1 1 • OH + • OH→H 2 O 2 1 1 5.5×10 9 1 1 R130 [ 52 52 ] 1 1 2 • NO 2 + H 2 O→HNO 2 + HNO 3 1 1 8.4×10 7 1 1 R019 [ 32 32 ] 1 1 • OH + • O − →HO 2 − 1 1 7.61×10 9 1 1 R131 [ 32 32 ] 1 1 • OH+ HNO 2 → • NO 2 + H 2 O 1 1 2.6×10 9 1 1 R020 [ 32 32 ] 1 1 • OH + H 2 → • H + H 2 O 1 1 4.2×10 7 1 1 R132 [ 33 33 ] 1 1 • H + HNO 2 → • NO + H 2 O 1 1 4.5×10 8 1 1 R021 [ 32 32 ] 1 1 • OH + OH − → • O − + H 2 O 1 1 1.3×10 10 1 1 R133 [ 53 53 ] 1 1 • NO 3 + HNO 2 → • NO 2 + HNO 3 1 1 2.0×10 8 1 1 R022 [ 32 32 ] 1 1 • OH + H 2 O 2 →H 2 O+ • HO 2 1 1 2.7×10 7 1 1 R134 [ 39 39 ] 1 1 HNO 2 →H + + NO 2 − 1 1 4.6×10 6 1 1 R023 [ 32 32 ] 1 1 • OH + HO 2 − →OH − + • HO 2 1 1 7.5×10 9 1 1 R135 [ 39 39 ] 1 1 H + + NO 2 − →HNO 2 1 1 1.0×10 10 1 1 R024 [ 32 32 ] 1 1 • OH + H 2 O 2 + →H 3 O + + O 2 1 1 1.2×10 10 1 1 R136 [ 54 54 ] 1 1 HNO 3 à H + + NO 3 − 1 1 2.0×10 10 1 1 R025 [ 32 32 ] 1 1 • OH + HO 2 • →H 2 O + O 2 1 1 6.0×10 9 1 1 R137 [ 54 54 ] 1 1 H + + NO 3 − →HNO 3 1 1 4.4×10 8 1 1 R026 [ 32 32 ] 1 1 • OH + O 2 −• →OH - + O 2 1 1 8.0×10 9 1 1 R138 [ 21 21 ] 1 1 NO 3 − + O(P) →NO 2 − + O 2 1 1 2.2×10 9 1 1 R027 [ 32 32 ] 1 1 • O − + H 2 O→OH − + • OH 1 1 1.8×10 6 1 1 R139 [ 41 41 ] 1 1 • NO 2 + HO 2 →HNO 2 + O 2 1 1 1.8×10 9 1 1 R028 [ 32 32 ] 1 1 • O − + • O - →O 2 2− 1 1 9.0 ×10 8 1 1 R140 [ 41 41 ] 1 1 • NO 2 + O 2 − →NO 2 − + O 2 1 1 4.5×10 9 1 1 R029 [ 32 32 ] 1 1 • O − + H 2 →OH − + H • 1 1 8.0×10 7 1 1 R141[This work] 1 1 NO→ NO(g) 1 1 9.0×10 −2 1 1 R030 [ 32 32 ] 1 1 • O − + H 2 O 2 →O 2 −• + H 2 O 1 1 7.08×10 8 1 1 R141 [ 30 30 ] 1 1 NO 3 + H 2 O→HNO 3 + • OH 1 1 3.0×10 2 1 1 R031 [ 32 32 ] 1 1 • O − + HO 2 − →O 2 − + OH − 1 1 4.0×10 8 1 1 R142 [ 30 30 ] 1 1 NO 3 + H 2 O 2 →HNO 3 + • HO 2 1 1 7.1×10 6 1 1 R032 [ 32 32 ] 1 1 • O − + O 2 →O 3 − 1 1 3.6×10 9 1 1 R143 [ 30 30 ] 1 1 NO 3 + HO 2 →HNO 3 + O 2 1 1 3.0×10 9 1 1 R033 [ 32 32 ] 1 1 • O − + O 2 •− →2OH − + O 2 1 1 6.0×10 8 1 1 R201 [ 27 27 ] 1 1 Pu(IV) + e aq − →Pu(III) 1 1 1.0×10 11 1 1 R034 [ 38 38 ] 1 1 HO • 2 →O 2 •− + H + 1 1 6.62×10 5 1 1 R202 [ 27 27 ] 1 1 Pu(IV) + • H →Pu(III) + H + 1 1 1.0×10 8 1 1 R035 [ 38 38 ] 1 1 O 2 •− + H + →HO • 2 1 1 4.52×10 10 1 1 R203 [ 27 27 ] 1 1 Pu(VI) + e aq − →Pu(V) 1 1 6.5×10 10 1 1 R036 [ 38 38 ] 1 1 H 2 O 2 →H + + HO 2 − 1 1 6.49×10 −2 1 1 R204 [ 27 27 ] 1 1 Pu(VI) + • H →Pu(V) + H + 1 1 6.5×10 7 1 1 R037 [ 38 38 ] 1 1 H + + HO 2 →H 2 O 2 1 1 4.52×10 10 1 1 R205 [ 27 27 ] 1 1 Pu(VI)+HNO 2 →Pu(V)+ • NO 2 +H + 1 1 1×10 −1 1 1 R038 [ 38 38 ] 1 1 H 2 O 2 →H 2 O + (1/2) O 2 1 1 8.29×10 −8 1 1 R206 [ 27 27 ] 1 1 Pu(V) + • NO 2 →Pu(VI) + NO 2 - 1 1 1.9×10 4 1 1 R101 [ 32 32 ] 1 1 NO 3 − + e aq − →NO 3 2− 1 1 9.7×10 9 1 1 R207 [ 27 27 ] 1 1 Pu(IV)+HNO 2 →Pu(III)+ • NO 2 +H + 1 1 7×10 −3 1 1 R102 [ 33 33 ] 1 1 NO 3 − + H • →OH − + • NO 2 1 1 1.4×10 6 1 1 R208 [ 27 27 ] 1 1 Pu(III) + • NO 2 + H + →Pu(IV) + HNO 2 1 1 2.5×10 3 1 1 R103 [ 39 39 ] 1 1 HNO 3 + H • →H 2 + • NO 3 1 1 1.0×10 7 1 1 R209 [ 27 27 ] 1 1 Pu(V) + • NO 3 + H + →Pu(VI) + HNO 3 1 1 1×10 9 1 1 R104 [ 32 32 ] 1 1 NO 3 2− + H + →OH − + • NO 2 1 1 2.0×10 10 1 1 R210 [ 27 27 ] 1 1 Pu(III) + • NO 3 + H + →Pu(IV) + HNO 3 1 1 2.5×10 8 1 1 R105 [ 32 32 ] 1 1 NO 3 2− + H 2 O→2OH − + • NO 2 1 1 5.6×10 4 1 1 R211 [ 27 27 ] 1 1 Pu(IV) + • NO 3 + H + →Pu(V) + HNO 3 1 1 6×10 2 1 1 R106 [ 32 32 ] 1 1 NO 3 2− + O 2 → • NO 3 − + • O 2 − 1 1 2.4×10 8 1 1 R212 [ 27 27 ] 1 1 Pu(V) + Pu(IV) →Pu(VI) + Pu(III) 1 1 1.2×10 1 1 1 R107 [ 32 32 ] 1 1 NO 3 2− + • OH → • NO 3 − + OH − 1 1 3.0×10 9 1 1 R213 [ 27 27 ] 1 1 Pu(VI) + Pu(III) →Pu(V) + Pu(IV) 1 1 2.4×10 1 1 1 R108 [ 20 20 ] 1 1 HNO 3 + • OH → • NO 3 + H 2 O 1 1 1.4×10 8 1 1 R214 [ 27 27 ] 1 1 Pu(V) + Pu(III) →2Pu(IV) 1 1 3.5×10 0 1 1 R109 [ 32 32 ] 1 1 • NO 3 + • OH → • NO 2 + • HO 2 1 1 2.9×10 7 1 1 R215 [ 27 27 ] 1 1 2Pu(IV) →Pu(V) + Pu(III) 1 1 3×10 −6 1 1 R110 [ 50 50 ] 1 1 2 • NO 3 →2 • NO 2 + O 2 1 1 7.9×10 5 1 1 R216 [ 27 27 ] 1 1 2Pu(V) →Pu(IV) + Pu(VI) 1 1 1×10 −2 1 1 R111 [ 32 32 ] 1 1 NO 2 − + • OH→ • NO 2 + OH − 1 1 9.6×10 9 1 1 R217 [ 27 27 ] 1 1 Pu(IV) + H 2 O 2 →Pu(III) + • HO 2 +H + 1 1 2×10 3 1 1 R112 [ 32 32 ] 1 1 NO 2 − + • NO 3 → • NO 2 + NO 3 − 1 1 1.2×10 9 1 1 R218 [ 27 27 ] 1 1 Pu(VI) + H 2 O 2 →Pu(V) + • HO 2 + H + 1 1 3×10 −2 The numerical calculations in the simulation for the radiolysis of aqueous nitric acid were conducted with MCPA FACSIMILE [ 29 29 ]. All the primary yields and reaction rate constants keep invariant while the dose rate, pH value and nitrate ion concentration vary. 3. Results and Discussions 1 3.1. Time dependence of absorbance curve 2 The typical absorbance curve for the aqueous nitric acid solution of 238 Pu as a function of irradiation time (up to 300 min) is illustrated in the upper part of Fig. 1 Fig. 1 , with the wavelength from 340 to 500 nm. The absorbance peaks of Pu(III), Pu(IV), Pu(V) and Pu(VI) appear at 600, 476, 569 and 831 nm, respectively [ 43 43 - 45 45 43-45 ]. Their corresponding molar absorption coefficients are 36, 68, 20, and 158 M −1 ·cm −1 , respectively (The molar absorption coefficients of Pu vary with the HNO 3 concentration. Only typical values are shown here.). In Fig. 1 Fig. 1 , the absorbance peak of Pu(IV) is distinct and has little variation. The peaks for Pu(V) and Pu(IV) are not observed and that for Pu(III) is quite weak (not shown in Fig. 1 Fig. 1 ). This indicates that Pu ions are mostly tetravalent during experiment. The absorbance curve changes obviously around 371 nm, so the area near 371 nm is enlarged in the lower part of Fig. 1. Fig. 1. One of the five absorbance peaks of nitrous acid is located at 371 nm. As the concentration of Pu(IV) is nearly invariant, the change of absorbance around 371 nm can be completely attributed to the production of nitrous acid. The equilibrium constant for dissociation of nitrous acid is 10 −3.27 M [ 46 46 ]. The solution is strongly acidic in experiment, with the concentration of HNO 3 ranging from 1.0 to 5.0 M. So nitrous acid hardly dissociates. The absorbance at 371 nm represents the changes in nitrous acid concentration, noted as c (HNO 2 ). The absorbance A = εcl, where ε is the molar absorption coefficient, c is the concentration, and l is the optical path. The molar absorption coefficient of nitrous acid is taken from Ref.[ 47 47 ]: ε (HNO 2 , 371 nm) = 60.46 M −1 ·cm −1 and ε (NO 2 − , 371 nm) = 16.18 M −1 ·cm −1 . 3.2 Valence of Pu 2 The simulated oxidation states of Pu as a function of irradiation time is shown in Fig. 2. Fig. 2. It’s obvious that the valences of Pu hardly change during experiment. Pu is predominantly tetravalent. This is in agreement with experimental results. What should be emphasized is that, the radiolysis behavior of aqueous nitric acid takes place as early as PuO 2 powder is suspended in stock solution (0.35 M nitric acid). However, the reactions following the complete dissolution of PuO 2 powder in aqueous nitric acid is complex and unpredictable as the inhomogeneity of solution and impurities are involved. As shown in experiment, Pu is mainly tetravalent in the stock solution, and the valences of Pu have little change during experiment. For simplicity, Pu is assumed to be tetravalent totally. Besides, the variation instead of amount of c (HNO 2 ) is concerned in this work. Without loss of generality，the c (HNO 2 ) in stock solution is set to be zero in simulation. 3.3. Dose-rate effect 2 Since the radiation energy is almost totally absorbed by the solution, the absorbed dose is proportional to the concentration of Pu. The dose-rate effect on the production of nitrous acid is investigated by varying the concentration of Pu. Fig. 3 Fig. 3 shows the experimental and theoretical results of absorbance at 371 nm as a function of the irradiation time. The dose rate of solution using 1 g/L Pu as radiation source is 32.5 Gy·min −1 . (The exact value of dose rate varies with the solution density. Such effect is taken into consideration in our calculation.) This value is calculated from the half-life of 238 Pu (87.7 years) and the averaged energy of α particles (5.5 MeV) assuming that all the energy of α particles are absorbed by the solution. The calculated value has good consistency with the measured data using scintillation spectrometry within the experimental time range. The experimental method was described in our previous work [ 48 48 ]. Thus the kinetic model is suitable to describe the α-radiolysis of nitric acid aqueous solution using 238 Pu as the radiation source. The production of nitrous acid increases with increasing dose obviously. 3.4. Variation of the concentration of nitric acid 2 Fig. 4 Fig. 4 shows the time dependence of absorbance at 371 nm with different concentrations of nitric acid. The calculation results fit fairly well the experimental data. The nitrous acid production increases with the concentration of nitric acid. Together with the results about dose-rate effect (shown in Fig. 3 Fig. 3 ), one can see that the radical yield, instead of nitric acid concentration, is vital for the nitrous acid yield, within the range of nitric acid concentration and dose rate in this experiment. Thus the roughly overall reaction about nitrous acid yield, namely NO 3 − ↔NO 2 − + O • , is far from being desired. Besides, as the nitric acid concentration rises, the electron fraction of each component varies, hence the changes in radical yield. Such change of radical yield has little effect on the nitrous acid yield, either. That can be roughly explained by the competition among the radiolysis of pure water, un-dissociated nitric acid, and nitrate ion. 3.5. Variation of concentration of nitrate ion 2 Both the concentrations of molecular nitric acid and nitrate ion change with c (HNO 3 ). Fig. 5 Fig. 5 shows the measured and simulated absorbance at 371 nm with different c (NaNO 3 ) as a function of irradiation time, at c (HNO 3 )=1.0 M and c (Pu)=2.9 g/L. The two results agree well with each other. It is seen that, the yield of HNO 2 increases with c (NO 3 − ) slowly. Since the radiolysis of pure water produces more radicals, the increase of c (NO 3 − ) only increase the yield of radicals slightly. Thus the production of HNO 2 has an unconspicuous increase with c (NaNO 3 ) from 1.41 to 3.53 M. 3.6. Effect of redox reactions about Pu 2 The influences of the reactions involving Pu (R201–R218 in Table 2 Table 2 ) on the production of HNO 2 were analyzed. In order to check the effects of R201–R218, Fig. 6(a) Fig. 6(a) shows the simulated absorbance at 371 nm with the model that excludes certain reactions, at c (HNO 3 )=3.0 M and c (Pu)=1.7 g/L. Without all the reactions about Pu, the curve deviates significantly from the experimental data, indicating the importance of reactions involving Pu. Then the important reactions among all the 18 reactions involving Pu were picked up. As shown in Fig. 6(a) Fig. 6(a) , without R217, the production of HNO 2 is much less than the experimental data and close to results without all reactions involving Pu. R217 is the vital reaction, which is expressed as: <math display="block" id="M10"><mrow><mtext>Pu</mtext><mrow><mo>(</mo><mrow><mtext>IV</mtext></mrow><mo>)</mo></mrow><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>2</mtext></msub><mtext>→Pu</mtext><mrow><mo>(</mo><mrow><mtext>III</mtext></mrow><mo>)</mo></mrow><msup><mtext>+</mtext><mtext>•</mtext></msup><msub><mrow><mtext>HO</mtext></mrow><mtext>2</mtext></msub><msup><mrow><mtext>+ H</mtext></mrow><mtext>+</mtext></msup><mtext>, </mtext><mi>k</mi><mtext>=2</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>3</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> Pu(IV) is reduced to Pu(III) by H 2 O 2 . H 2 O 2 also consumes HNO 2 rapidly through the reaction: <math display="block" id="M11"><mrow><msub><mrow><mtext>HNO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>2</mtext></msub><msub><mrow><mtext>→ONOOH+H</mtext></mrow><mtext>2</mtext></msub><mtext>O, </mtext><mi>k</mi><mtext>=7</mtext><mtext>.17</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mrow><mtext>5 </mtext></mrow></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> Thus R217 benefits the production of HNO 2 . Pu(III) produced in R217 can be oxidized to Pu(IV) in R208: <math display="block" id="M12"><mrow><mtext>Pu</mtext><mrow><mo>(</mo><mrow><mtext>III</mtext></mrow><mo>)</mo></mrow><msup><mtext>+</mtext><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>2</mtext></msub><mtext>→Pu</mtext><mrow><mo>(</mo><mrow><mtext>IV</mtext></mrow><mo>)</mo></mrow><msub><mrow><mtext>+NO</mtext></mrow><mtext>2</mtext></msub><msup><mrow/><mtext>-</mtext></msup><mtext>, </mtext><mi>k</mi><mtext>=2</mtext><mtext>.5</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>3</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> Here • NO 2 was reduced to NO 2 − . This reaction, together with its reverse reaction R207, also has obvious influence on the production of HNO 2 . All the other reactions about Pu, such as the disproportionate/symproportionate reactions (R212– R216), are of either too small reactant concentrations or reaction rate constants. Their contributions are negligible (less than 0.1 %), and are not plotted in Fig. 6(a) Fig. 6(a) . For further discussion about the effect of reactions involving Pu, c (Pu) react is defined as the concentration of Pu involved in the chemical reactions. The simulation results of different c (Pu) react are plotted in Fig. 6(b) Fig. 6(b) , at c (HNO 3 )=3.0 M and c (Pu) =1.7 g/L. The yield of HNO 2 increases with c (Pu) react at c (Pu) react < 0.5 g/L, and keeps nearly invariant at c (Pu) react > 0.5 g/L. This indicates that, Pu(IV) should be treated as catalyst instead of reactant/oxidant for the production of HNO 2 on the whole. Its effect can be represented roughly as: <math display="block" id="M13"><mrow><msub><mtext>H</mtext><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>2</mtext></msub><msub><mrow><mtext>+NO</mtext></mrow><mtext>2</mtext></msub><mover><mo>→</mo><mrow><mtext>Pu(IV)</mtext></mrow></mover><msub><mrow><mtext>HNO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+HO</mtext></mrow><mtext>2</mtext></msub></mrow></math> Besides, the reaction rate constants of Pu depend on c(HNO 3 ) [ 27 27 , 28 28 ]. However, the rate constant for the most important reaction, Pu(IV) + H 2 O 2 →Pu(III) + • HO 2 +H + , is invariant with c(HNO 3 ). Therefore, the variation of rate constants with c (HNO 3 ) has little effect on the yield of HNO 2 . 3.7. Mechanism for the production of nitrous acid 2 The intrinsic mechanism of nitrous acid generation and consumption was comprehensively investigated. With this model, the reaction rates for the reactions that directly generate or consume nitrous acid was calculated at c (HNO 3 )=3.0 M and c (Pu)=1.7 g/L. The results of the six most important reactions for generation and consumption of HNO 2 are shown in Fig 7 Fig 7 . The rates of other reactions are more than one order of magnitudes lower than that of the selected ones. The most important reactions that contribute to generation of nitrous acid are given as below: <math display="block" id="M14"><mrow><msup><mrow><mtext>R127: </mtext></mrow><mtext>•</mtext></msup><msup><mrow><mtext>NO+</mtext></mrow><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>O→2HNO</mtext></mrow><mtext>2</mtext></msub><mtext>, </mtext><mi>k</mi><mtext>=1</mtext><mtext>.8</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>1</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> <math display="block" id="M15"><mrow><msup><mrow><mtext>R130: 2</mtext></mrow><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>O→HNO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+HNO</mtext></mrow><mtext>3</mtext></msub><mo>,</mo><mtext> </mtext><mi>k</mi><mtext>=8</mtext><mtext>.4</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>7</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> <math display="block" id="M16"><mrow><msub><mrow><mtext>R115: ONOOH+H</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>O→H</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>2</mtext></msub><msub><mrow><mtext>+HNO</mtext></mrow><mtext>2</mtext></msub><mtext>, </mtext><mi>k</mi><mtext>=5</mtext><mtext>.4</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>0</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> And the three reactions that dominate the consumption of nitrous acid are also collected here: <math display="block" id="M17"><mrow><msub><mrow><mtext>R126: 2HNO</mtext></mrow><mtext>2</mtext></msub><msup><mtext>→</mtext><mtext>•</mtext></msup><msup><mrow><mtext>NO+</mtext></mrow><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O, </mtext><mi>k</mi><mtext>=1</mtext><mtext>.34</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>1</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> <math display="block" id="M18"><mrow><msub><mrow><mtext>R129: HNO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+HNO</mtext></mrow><mtext>3</mtext></msub><msup><mtext>→</mtext><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>2</mtext></msub><msup><mtext>+</mtext><mtext>•</mtext></msup><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub><msup><mrow><mtext>+2H</mtext></mrow><mtext>+</mtext></msup><mtext>, </mtext><mi>k</mi><mtext>=4</mtext><mtext>.5</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mrow><mtext>-2</mtext></mrow></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> <math display="block" id="M19"><mrow><msub><mrow><mtext>R114: H</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>2</mtext></msub><msub><mrow><mtext>+HNO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>→ONOOH+H</mtext></mrow><mtext>2</mtext></msub><mtext>O, </mtext><mi>k</mi><mtext>=7</mtext><mtext>.17</mtext><mo>×</mo><msup><mrow><mtext>10</mtext></mrow><mtext>5</mtext></msup><msup><mtext>M</mtext><mrow><mtext>-1</mtext></mrow></msup><mo>·</mo><msup><mtext>s</mtext><mrow><mtext>-1</mtext></mrow></msup></mrow></math> The two sets of reactions are reverse reactions. It is seen that, NO 2 , NO and H 2 O 2 are the vital intermediate products for production of HNO 2 . Obviously, these reactions and intermediate productions in the PUREX process is paramount in that they actually show us how to manipulate the concentration of nitrous acid. Although the reaction constant rates in Eqs.(14)–(19) differ by 7 orders of magnitude, their contribution to the generation/consumption of HNO 2 is comparable, as there are significant difference between the concentrations of reactant. 4. Conclusion 1 In this work, the α-radiolysis of aqueous nitric acid with 238 Pu as an irradiation source is studied, and a kinetic model is established accordingly. The influence of dose rate, nitric acid concentration, and nitrate ion concentration on the yield of nitrous acid are investigated experimentally. Our results have implications for the practical reprocessing of spent fuel, as the nitric acid concentration used in the experiment purposely simulates that in PUREX process. 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We also thank Prof. Chung-King Liu (USTC) and Dr. Han-Qin Weng (USTC) for revising this manuscript. 10.1007/s41365-017-0200-4 This work was partly supported by the National Natural Science Foundation of China (No. 21377122). 2016-04-23 2016-07-25 2016-08-12 2017-03-03 2017-04-01 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2017 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-017-0200-4 http://dx.doi.org/10.1007/s41365-017-0200-4 . 2017 Nuclear Science and Techniques 04 28